Not to mention all the run off from the streets!If you are after a little open air swimming maybe go up to the ponds on Hampstead heath.

Open-air swimming is internationally famous on Hampstead Heath with the Ladies’ Pond, the Men’s Pond, the Mixed Pond and the Parliament Hill Lido. The Ladies’ and Men’s Ponds are unique in the United Kingdom in being the only life-guarded open-water swimming facilities open to the public every day of the year.